Food Science and Technology Research
Online ISSN : 1881-3984
Print ISSN : 1344-6606
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Original papers
Comparative Study on Phenolic Profiles and Antioxidant Activity of Litchi Juice Treated by High Pressure Carbon Dioxide and Thermal Processing
Lei LiuQinshuai ZengRuifen ZhangZhencheng WeiYuanyuan DengYan ZhangXiaojun TangMingwei Zhang
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2015 Volume 21 Issue 1 Pages 41-49

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Abstract

The present study compared physicochemical parameters, total phenolic and total flavonoid contents, individual phenolics, and antioxidant activity of litchi juice processed by high pressure carbon dioxide (HPCD), high temperature and ultra high temperature treatments. The results showed that HPCD can retain color of litchi juice better than thermal processing (p < 0.05), while three processing methods had no influence on pH and total soluble solid in litchi juice. Compared to thermal processing, HPCD processed litchi juice had more total phenolics, total flavonoid and individual phenolics including rutin, (−)-epicatechin and chlorogenic acid (p < 0.05). Furthermore, the antioxidant activity of litchi juice was analyzed by 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric reducing antioxidant power (FRAP) and 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) assays. The result indicated that HPCD processing can preserve the antioxidant activity better than thermal processing (p < 0.05). This study suggested that HPCD could be used as an alternative of traditional thermal processing to produce more fresh fruit juice.

Introduction

Numerous studies have demonstrated that increased consumption of fruit and vegetable is positively associated with reduced risk of developing chronic diseases, such as cardiovascular disease, diabetes, cancer and Alzheimers's disease (Liu, 2003; Pappas and Schaich, 2009). These health benefits have been attributed in part to the unique phytochemicals of fruit and vegetable (Eberhardt et al., 2000; Liu, 2007).

Litchi (Litchi chinensis Sonn.) is a subtropical to tropical fruit of high commercial value in international trade. It becomes one of popular fruit due to its delicious taste and healthy benefits. Phenolic compounds are known as the main active components in litchi fruit, having various bioactive activities, including antioxidant, anticancer, and immunomodulatory activities (Yang et al., 2012; Zhao et al., 2007). Phenolic compounds are also closely related with sensory quality. Until now, most of published literature on the phenolic content and antioxidant activity of litchi fruit focused on its pericarp and seeds (Jiang et al., 2013; Li et al., 2012b; Nagendra Prasad et al., 2009). Our recent study reported phenolic profiles and antioxidant activity of litchi pulp from different cultivars (Zhang et al., 2013). However, little information is available about the phenolic profiles and antioxidant activity of litchi juice.

Litchi is a seasonal fruit and susceptible to spoilage. Therefore, further processing is desirable to extend shelf-life and meet the market demands. Litchi is usually processed into dried fruit or juice. Traditionally, high temperature (HT) short time process is applied in fruit juice processing to prevent microbial growth and to inactivate endogenous enzymes that can cause undesirable quality change (Krebbers et al., 2003). Compared to the traditional thermal processing, ultra high temperature (UHT) system which is often used to process milk is a relatively new preservation technique with higher temperature and shorter time (Xu et al., 2010). Unfortunately, thermal processing can simultaneously result in the loss of nutritional components and change of fruit juice color (Charles et al., 2007; Hsu, 2008). Therefore, in order to satisfy the consumer's demand for fresh-like food products with a high nutritional and sensory quality, high pressure carbon dioxide (HPCD) processing has been rapidly developed as a non-thermal processing method for liquid foods in recent years. Many studies have already revealed that this technique can effectively inactivate microorganisms and enzymes in fruit and vegetable juice, such as apple, orange, watermelon, banana, and carrot (Gasperi et al., 2009; Fabroni et al., 2010; Liu et al., 2012; Yu et al., 2013; Zhou et al., 2009). Furthermore, it has been found that HPCD processing retained the fresh-like sensory, nutritional, and physical properties of many liquid foods without comprising on quality caused by heating (Chen et al., 2012, Damar and Balaban, 2006; Kincal et al., 2006). Recent studies have also suggested that HPCD processing could not only inactivate microorganisms in litchi juice efficiently including aerobic bacteria, yeasts and moulds, but also maintain the concentration of polyphenols in the juice (Guo et al., 2011; Li et al., 2012a). But this study did not systematically analyze the phytochemicals and antioxidant activity of litchi juice. In addition, it remains largely unknown how HPCD and thermal processing affect the phnolic compositions, including flavonoids, and antioxidant activity of litchi juice. Therefore, the objectives of the present study were to compare the effects of HPCD and thermal processing (HT and UHT) on phenolic profiles and antioxidant activity of litchi juice. At the same time, the physicochemical properties including browning index, pH and the soluble content of litchi juice were also studied after processing.

Materials and Methods

Chemicals and reagents    Gallic acid, chlorogenic acid, rutin, (−)-epicatechin, (+)-catechin, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), Folin-Ciocalteu's phenol reagent, 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,4,6-tripyridyl-s-triazine (TPTZ) and 2,2′-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) were purchased from Sigma Chemical Co. (Suwanee, GA, USA). All solvents used in chromatography were of HPLC grade and other chemicals were of analytical reagent grade.

Preparation of litchi juice    The fresh litchi cultivar (c.v. Heiye) was provided by Fruit Tree Research Institute, Guangdong Academy of Agricultural Sciences. The fresh fruits were peeled and deseeded immediately after harvest. The litchi pulp was blanched for 30s in 100°C water followed by squeezing. The squeezed juice was centrifuged at 5000 rpm for 5 min. Based on the methods that have been used to the sterilization of litchi juice in southern China, the obtained juice (control juice) was then sterilized by three methods as follows: HPCD (pressure, 8 MPa, 36°C, 120s), HT (100°C, 60s), and ultra UHT (134°C, 4s). The treated juice was stored at 4°C for analysis.

HPCD processing of litchi juice    According to the system of Liao et al. (2007), HPCD system was designed and purchased from Hangzhou Huali Pump Co. LTD. (Zhejiang, China). A stainless steel vessel with a volume of 1000 mL was manufactured to withstand a pressure of 48 MPa. The vessel temperature was maintained by a thermostatic bath, and was monitored by a thermocouple inside the vessel. The liquid CO2 (Guangzhou Longou Co., China) of 99.5% purity was pumped into the vessel by a plunger pump with a maximum flow rate of 50 L/h and a maximum pressure of 50 MPa. All the data of temperature and pressure were displayed on a control panel. Further details of the equipment and the procedure are described in Yu et al. (2013).

After the equipment pipe and vessel were thoroughly rinsed by alcohol (75%) and aseptic water successively, a 200 mL of litchi juice was put into the vessel and the temperature of thermostatic bath was maintained at 36°C. Then, the vessel was pressurized by CO2 for 2 min under the pressure of 8 MPa. The depressurization was performed by releasing CO2 from the vessel. After HPCD treatment, the treated samples were filled into sterile bottles and stored at 4°C for further analysis.

UHT processing of litchi juice    This experiment was performed using a Mini T/T UHT system (Wodi Technology Co. LTD., Shanghai, China), provided with a plate heat exchanger and a recirculating glycol chiller. The tested temperature was at 134°C for 4s. The treated juice was immediately cooled in cold water, and then stored at 4°C for further analysis.

HT processing of litchi juice    A cup contained 200 mL litchi juice was heated in a boiling water bath (100°C) for 60s. The treated juice was immediately cooled in cold water bath, and then stored at 4°C for further analysis.

Determination of browning degree    Based on the previous report (Roig et al., 1999), browning degree of litchi juice was measured by their absorbance at 420 nm using spectrophotometer with 1 cm path length cell (Shimadzu Inc., Kyoto, Japan).

Determination of pH    The pH values were measured at 20°C with a pH meter (Thermo Fisher Scientific, Inc, MA, U.S.A), which was calibrated with pH 4.0 – 7.0 buffer.

Determination of total soluble solid    Total soluble solid of litchi juice was determined as °Brix using a WAY-2S digital Abbe Refractionmeter (Shanghai Precision and Scientific Instrument Co., Shanghai, China) at 20°C.

Extraction of total phenolics    Total phenolics were extracted according to a previous method with slight modification (Xu et al., 2008). Briefly, 20 mL of the above litchi juice was mixed with 40 mL of 80% methanol in a Philips blender for 5 min. The filtrates were concentrated under vacuum at 45°C until approximately 90% of the filtrate had been evaporated. The concentrated extraction was then recovered with distilled water to a final volume of 5 mL and then stored at −40°C until use.

Determination of total phenolic contents    Total phenolic contents were measured by the Folin-Ciocalteu (FC) colorimetric method described previously by Dewanto et al. (2002). Briefly, the above extract (125 µL) was diluted with distilled water (0.5 mL) and subsequently mixed with FC reagent (125 µL). After 6 min, 7% Na2CO3 (1.25 mL) was added into the solution, and the solution was diluted to a final volume of 3 mL. The reaction solution was incubated in dark for 90 min, and the absorbance was measured at 760 nm using a Shimadzu UV-1800 spectrometer (Shimadzu Inc., Kyoto, Japan). Gallic acid was used as a standard, and the results were expressed as microgrammes of gallic acid equivalents (GAE) per milliliter of processed litchi juice.

Determination of total flavonoids contents    The total flavonoids contents of litchi juice were measured according to the colorimetric method (Dewanto et al., 2002). The above extract (250 µL) was mixed with distilled water (1.25 mL), and subsequently with 5% NaNO2 solution (75 µL). After 6 min, 10% AlCl3·6H2O (150 µL) was added to the solution and allowed to react for 5 min. Then, 1 M NaOH (0.5 mL) was added, and the mixture was diluted to a final volume of 2.5 mL with distilled water. The absorbance of the mixture was immediately measured at 510 nm using a Shimadzu UV-1800 spectrometer. (+)-catechin was used as a standard, and the results were expressed as microgrammes of (+)-catechin equivalents (CE) per milliliter of litchi juice.

Determination of phenolic compositions    Quantitative analysis of phenolic compositions was performed using an Agilent 1200HPLC system (Waldbronn, Germany) equipped with an Agilent 1200 series VWD detector and autosampler. An Agilent Zorbox SB-C18 column (250 mm × 4.6 mm id, 5 µm, Palo Alto, CA, USA) was used at column temperature 30°C. The mobile phase consisted of 0.4% aqueous solution of acetic acid (solution A) and acetonitrile (solution B) using the following gradient program: 0 – 40 min, solution B 5 – 25%; 40 – 45 min, solution B 25 – 35%; 45 – 50 min, solution B 35 – 50%. Other chromatographic conditions included a constant flow rate of 1.0 mL/min, an injection volume of 20 µL, a run time of 50 min, and detection wavelength of 280 nm. Identification of each peak was primarily based on comparison of their retention times with the known authentic standards. The percent recovery of these phenolics was from 95.8 to 98.2%. Prior to analysis, all of the samples were filtered through a 0.25 µm membrane filter (Millipore, Billerica, MA, USA).

DPPH radical scavenging activity assay    The DPPH radical scavenging capacities of litchi juice were measured according to the previous report (Zhang et al., 2011). Briefly, 200 µL of serially diluted samples or methanol (control) were added to 2.8 mL of methanolic solution of 70 µM DPPH. The absorbance was determined at 517 nm after 30 min of incubation in the dark at room temperature. The percentage of DPPH radical scavenging activity (%) of the sample was calculated as follows: DPPH radical scavenging activity (%) = (Abscontrol − Abssample) × 100/Abscontrol, where Abscontrol is the absorbance of control (DPPH solution) and Abssample is the absorbance of test sample (DPPH solution plus sample extract). EC50 value is the effective concentration that could scavenge 50% of the DPPH radicals. EC50 was calculated by constructing the percentage of DPPH scanvenging versus log (extract concentration expressed as the weight of litchi pulp) curves (mg/mL).

Ferric reducing antioxidant power (FRAP) assay    The FRAP assay was performed according to the previous method (Benzie and Strain, 1996). Fresh working FRAP reagent was prepared daily by mixing 25 mL acetate buffer (300 mM, pH 3.6), 2.5 mL TPTZ solution (10 mM TPTZ in 40 mM HCl), and 2.5 mL of 20 mM FeCl3·6H2O solution. The reagent was warmed to 37°C. Litchi juice extract or distilled water as blank (200 µL) were allowed to react with 2.8 mL of the working reagent for 30 min in dark at room temperature. The absorbance was measured at 593 nm with a Shimadzu UV-1800 spectrometer. Trolox was chosen as a reference standard, and the FRAP antioxidant activity was expressed as microgrammes of trolox equivalents (TE) per milliliter of litchi juice.

ABTS radical cation decolorisation assay    The free radical scavenging capacity of litchi juice was evaluated using the ABTS radical cation decolourisation assay (Re et al., 1999), which is based on the reduction of ABTS•+ radicals by antioxidants of the juice tested. The ABTS•+ solution was prepared with final concentration of 7 mM ABTS and 2.45 mM potassium persulfate.

The mixture was left in the dark at room temperature for 12 h before use. For the study, the ABTS•+ solution was diluted in deionized water or ethanol to an absorbance of 0.7 ± 0.02 at 734 nm, verified with a UV-visible spectrophotometer. Sample solution (50 µL) was added to working solution (1 mL), and shaked vigorously for 30 s. The absorbance was measured at 734 nm after 30min of incubation. Trolox was used as a reference standard, and the result was expressed as microgrammes of trolox equivalents (TE) per milliliter of litchi juice.

Statistical analysis    Data were expressed as mean±SD for triplicate determinations of each sample. The data were analyzed using SPSS13.0 software (SPSS Inc. Chicago, IL, USA) for one-way ANOVA and SNK test analysis, and the level for a significant difference was set at p < 0.05.

Results and Discussion

Physicochemical parameters of litchi juice processed by different methods    Table 1 shows physicochemical parameters of litchi juice processed by different methods. The browning degree of the tested juice was 0.08 (control), 0.10 (HPCD), 0.19 (HT), 0.20 (UHT), respectively. HPCD processing almost had no influence on the browning degree of litchi juice. This suggested that HPCD processing was good for the retention of litchi juice color. HT and UHT processing significantly increased the browning degree of litchi juice (p < 0.05), and there were no significant difference between the two processing methods groups. The result indicated that high temperature thermal processing was deleterious for the color of litchi juice. The browning reaction mainly caused by the degradation of ascorbic acid, oxidation of phenolic compounds and formation of 5-hydroxymethylfurfural under the condition of high temperature (Damasceno et al., 2008; Roiga et al., 1999), whereas the exclusion of oxygen and relative low temperature in HPCD processing could prevent the reactions in litchi juice.

Table 1. Physicochemical parameters of litchi juice processed by different methods
Treatment Browning degree pH Total soluble solid (°Brix)
Control 0.08 ± 0.01a 5.01 ± 0.10a 11.60 ± 0.04a
HPCD 0.10 ± 0.01a 4.98 ± 0.01a 11.51 ± 0.03a
HT 0.19 ± 0.02b 4.99 ± 0.20a 11.32 ± 0.11a
UHT 0.20 ± 0.02b 5.06 ± 0.11a 11.25 ± 0.04a

Values within columns followed by different letters are significantly different (p < 0.05).

pH is an important quality parameter in fruit juice. As shown in Table 1, litchi juice is a low acidic food with pH of 5.01. HPCD processing had no significant effect on pH of litchi juice, which was in consistent with the investigation of orange juice processed by HPCD (Kincal et al., 2006). However, Park et al. (2002) observed a decrease of pH from 6.5 to 4.4 in carrot juice after HPCD treatment. The decrease of pH was caused by carbonic acid formed by CO2 dissolving into the juice, which further dissociated to free hydrogen ions. In this study, the dissociation might be difficultly performed in acidic litchi juice (Yu et al., 2013). In addition, HT and UHT processing had also no influence on pH of litchi juice.

Total soluble solid content of litchi juice was also analyzed. HPCD, HT and UHT processing had no significant influence on the total soluble solid content of litchi juice (Table 1). The similar result was observed in orange juice treated by HPCD and thermal processing (Fabroni et al., 2010).

Total phenolics, phenolic compositions and total flavonoids of litchi juice processed by different methods

Effect of HPCD and thermal processing on the total phenolic contents of litchi juice was investigated. As shown in Fig. 1, the total phenolic contents of control juice were 404.2 µg GAE/mL, which was lower than that of litchi pulp. We previously reported that the total phenolic contents of litchi pulp of different cultivar ranged from 1.02 to 2.59 mg GAE/g. It was easily deduced that the phenolics of litchi pulp was lost during processing of litchi juice such as blanching and squeezing. HPCD processing had no influence on the total phenolic contents whereas HT and UHT treatments significantly decreased the total phenolic contents of litchi juice by 16.1% and 19.8%, respectively (p < 0.05). The result was in accord with previous studies. Ferrentino et al. (2009) reported that HPCD procession had no effects on the total phenolic contents in red grapefruit juice. Pozo-Insfran et al. (2006) also observed a similar result that HPCD processing had no influence on the total phenolic contents whereas HT treatment significantly decreased the total phenolic contents in muscadine grape juice. Greater phenolic degradation (48%) was found in apple juice pasteurized at 94°C (Noci et al., 2008). The decrease of phenolics presumably occurred due to the degradation and formation of byproducts from polyphenolics that can react with organic acid or carbonyl compounds such as furfurals during thermal processing (Pozo-Insfran et al., 2006).

Fig. 1.

Total phenolic contents of litchi juice processed by different methods (mean ± SD, n = 3)

Means with different letters are significantly different (p < 0.05).

Furthermore, the phenolic compositions of litchi juice were analyzed by HPLC. Three individual phenolics in litchi juice including (−)-epicatechin, rutin and chlorogenic acid were detected and quantified, accounting for 8.56, 5.78, and 2.41 µg/mL, respectively (Table 2). Guo et al. (2011) found that rutin, (−)-epicatechin and caffic acid were the main phenolic compounds in litchi juice, accounting for 8.74, 4.01, and 0.25 µg/mL, respectively. The discrepancy could be due to the different litchi varieties used in research. Compared to control juice, HPCD processing had no significant influence on the phenolic compositions, whereas HT and UHT treatments significantly decreased the three individual phenolic contents (p < 0.05). Moreover, UHT led to the lower content of (−)-epicatechin and chlogrogenic acid than HT treatment. However, the result is contrary to those found by Huang et al. (2013), who reported that HT processing (110°C, 8.6 s) significantly increased four individual phenolics in apricot nectar including caffeic acid, chlorogenic acid, (−)-epicatechin, and (+)-catechin. The difference may be due to the phenolic compounds in the apricot nectar being bound to other components such as polysaccharide and released by thermal treatment, while the phenolic compounds are free in the litchi juice, and therefore more susceptible to thermal processing.

Table 2. Phenolic composition of litchi juice processed by different methods
Treatment Rutin (µg/mL) (−)-epicatechin (µg/mL) Chlorogenic acid (µg/mL)
Control 8.56 ± 0.31b 5.78 ± 0.29c 2.41 ± 0.04c
HPCD 8.42 ± 0.24b 5.58 ± 0.27c 2.41 ± 0.03c
HT 6.95 ± 0.23a 4.22 ± 0.24b 2.07 ± 0.11b
UHT 6.91 ± 0.42a 3.67 ± 0.20a 1.96 ± 0.04a

Values within columns followed by different letters are significantly different (p < 0.05).

Effect of HPCD and thermal processing on the total flavonoid contents was also evaluated. As shown in Fig. 2, the total flavonoid contents were 34.1µg CE/mL in control juice. After processing, the total flavonoid contents were significantly decreased by 12.6% (HPCD), 25.8% (HT) and 40.1% (UHT), respectively (p < 0.05). Furthermore, the total flavonoid contents in HPCD group was higher than that of HT and UHT groups (p < 0.05), and UHT treatment led to the lowest contents of the total flavonoid (20.1µg CE/mL). Fabroni et al. (2010) also reported that total flavonoids of orange juice was slightly decreased after HPCD or thermal treatments. However, Xu et al. (2010) found that UHT processed soy milk exhibited significantly higher total flavonoids than raw soy milk. The possible explanation may be that thermal processing could release bound flavonoid components in soy milk, but the flavonoid compounds in litchi juice were free and unstable for thermal treatment.

Fig. 2.

Total flavonoid contents of litchi juice processed by different methods (mean ± SD, n = 3)

Means with different letters are significantly different (p < 0.05).

The three treatments have been proven to be efficient preservation method for fruit juice. However, our result showed that HT and UHT treatments caused significant decrease in the total phenolic content and flavonoid content. Because most of the enzymatic activity in litchi pulp was inactivated in blanching treatment, the reduction in total phenolics and flavonoids of litchi juice during the thermal processing could be mainly due to the non-enzymatic oxidation. Previous studies have suggested that phenolics could be oxidized to quinines that further react with other molecules such as amino acids and proteins in food system (Talcott and Howard, 1999; Terefe et al., 2013). Moreover, greater phenolic degradation (24%) was found in tomato puree treated at 128°C compared with those pasteurized at 98°C (12%), indicating that treatment temperature plays an important role in phenolic degradation (Perez-Conesa et al., 2009). In addition, the blanching treatment could not completely inactivate peroxidase and polyphenol oxidase, the residual enzymatic activity in litchi juice may also have contributed to the phenolic degradation during the thermal processing (Terefe et al., 2013).

Antioxidant activity of litchi juice processed by different methods

DPPH is a traditional method, which is sensitive enough to detect active components at low concentrations (Hsu, 2008). The EC50 value was defined as the effective concentration of juices necessary to cause 50% inhibition of DPPH radical, therefore, a lower EC50 value is associated with a higher radical scavenging activity. As shown in Fig. 3, the EC50 values of litchi juice were 29.9 (Control), 32.3 (HPCD), 37.1 (HT) and 39.6 µL/mL (UHT), respectively. It was easily found that HPCD processing had almost no influence on the DPPH radical scavenging activity of litchi juice, whereas HT and UHT processing significantly decreased DPPH radical scavenging activity (p < 0.05).

Fig. 3.

DPPH radical scavenging activities of litchi juice processed by different methods (mean ± SD, n = 3)

Means with different letters are significantly different (p < 0.05).

The reducing power of litchi juice was determined using a modified Fe3+ to Fe2+ reduction assay. As shown in Fig. 4, the FRAP value of the control juice was 679.2 µg TE/mL. Compared to control juice, HPCD, HT and UHT processing significantly decreased the FRAP value of the litchi juice by 9.7%, 21.6% and 32.5%, respectively (p < 0.05). Among the three processing HPCD treatment had the highest FRAP value, and UHT treatment led to the lowest FRAP value (p < 0.05).

Fig. 4.

FRAP value of litchi juice processed by different methods (mean ± SD, n = 3)

Means with different letters are significantly different (p < 0.05).

The ABTS radical cation discoloration test is another method widely used to assess antioxidant activity. The trend of ABTS radical scavenging activity was the same with FRAP. As shown in Fig. 5, the ABTS value of the control juice was 776.3 µg TE/mL. Compared to control juice, HPCD, HT and UHT processing significantly decreased the ABTS value of the litchi juice by 8.5%, 21.8% and 29.7%, respectively (p < 0.05). Among the three processing HPCD treatment had the highest ABTS value, and UHT treatment led to the lowest ABTS value (p < 0.05).

Fig. 5.

ABTS value of litchi juice processed by different methods (mean ± SD, n = 3)

Means with different letters are significantly different (p < 0.05).

The potential health benefit of phenolic compounds is mainly attributed to its antioxidant activity. The results of antioxidant activity measured by DPPH, FRAP and ABTS assay were consistent for the litchi juice treated with three different processing methods in this research. Our result demonstrated that HPCD processing could preserve the antioxidant activity of litchi juice better than thermal processing. This is mainly because HPCD processing could retain the phenolics and flavonoids content of litchi juice. The similar results were also found in blood orange juice and muscadine grape juice treated by HPCD and thermal processing (Ferrebtino et al., 2009; Pozo-Insfran et al., 2006). The retention of antioxidant activity of litchi juice was due to the exclusion of oxygen and relative low temperature during HPCD processing that prevented the degradation of antioxidant compounds such as phenolics and flavonoids in litchi juice. On the other hand, our recent study indicated that the antioxidant activity of litchi pulp was significantly correlated with their phenolics and flavonoids (Zhang et al., 2013). Therefore, it was easily deduced that the decrease of antioxidant activity of litchi juice induced by HT and UHT treatments could be due to the phenolic degradation during the thermal processing.

Conclusions

HPCD has been used as a relatively new non-thermal technology for pasteurization of liquid foods. Our previous study has reported that HPCD can inactivate microorganisms in litchi juice efficiently (Guo et al., 2011). Furthermore, this study compared the effects of HPCD and thermal processing on physicochemical parameter, total phenolics and total flavonoids contents, phenolic compositions, and antioxidant activity of litchi juice. The result showed that HT and UHT processing caused undesirable change in quality of litchi juice such as browning, phenolic degradation and the loss of antioxidant activity. Compared to thermal treatments, HPCD processing can not only retain the physiochemical and quality attributes of litchi juice, but also preserve phenolics, flavonoids, and antioxidant activity of litchi juice. Therefore, HPCD processing may be highly applicable for preparation of high quality and nutritious fruit juice, especially for juices containing heat labile phytochemicals, and antioxidant compounds instead of traditional thermal processing.

Acknowledgements    This research work was supported by Special Prophase Project on The National Basic Research Program of China (973 Project) (2012CB722904), the National Nature Science Foundation of China (31171680), Guangdong international cooperation project (2010B050600005) and Major Science and Technology Special Projects of Guangdong Province (2009A080209002).

References
 
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